Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
  • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
  • C23C8/20—Carburising
  • C23C8/22—Carburising of ferrous surfaces
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/02—Pretreatment of the material to be coated

Definitions

• the invention relates to a method for the carburization of a deep-drawn part or a stamped-bent part made of austenitic rustproof stainless steel and having a small wall thickness, which is usual for such parts, in at least some areas.
• Usual small wall thicknesses of deep-drawn parts and stamped-bent parts in the sense of the invention are beneath 2000 ⁇ m.
• Such stainless steel parts are manufactured from very thin sheet metals by means of tensile compression reshaping or stamping-bending and sometimes take very filigree structures.
• parts having a varying or constant wall thickness can be produced, whereby these ones then entirely comprise a wall thickness of less than 2000 ⁇ m or they have such a wall thickness in at least some areas.
• filigree items are used in the most different fields of technique, such as for example as bearing covers in gearboxes, valve seats in ABS systems or as sample carriers for hazardous substances in high-precision measurements and are subject to extreme mechanical, thermal and chemical stress.
• the demand for corrosion resistant materials comprising a high hardness is therefore accordingly high.
• US 2012/111454 thus shows for example a high temperature method for the carburization of rustproof steel ingots.
• carburization temperatures comprised between 760° C. and 1200° C. are used. Methods using such high temperatures cannot be used for the surface hardening of thin-walled deep-drawn and stamped-bent items, since they cause a thermal deformation of the partly very filigree structures and thus make them unusable.
• U.S. Pat. No. 6,461,448 shows a method for the carburization of a steel article, in which the said steel article is treated in a molten alkaline bath.
• Such an aggressive kind of treatment causes filigree items in the sense of the invention to get partially considerable corrosion damages because of the thin wall thickness, leading to a highly inhomogeneous surface layer.
• liquid treatments of filigree items lead to unsatisfying results because of an incomplete surface wetting.
• EP 0 678 589 B1 discloses a method for the carburization of an austenitic metal.
• a fluorine-based gas is applied to the metal.
• Fluorine-based gases are highly corrosive due to their reactivity and as a result they act aggressively on the surface of the metal. While the thus caused surface removal is even desired for articles having a high wall thickness and a corresponding high amount of material, this surface removal cannot be compensated in thin-walled deep-drawn and stamped-bent items and leads to the irreversible destruction of the article.
• the gases used there are highly toxic, highly corrosive and comprise properties that are highly endangering the environment and thus they place enormous requirements on the reactor to be used, the storage and the operational safety.
• the invention advantageously proposes a method comprising mild conditions which are adapted to the particularities of thin-walled deep-drawn and stamped-bent articles.
• the article is inserted into an oven for carrying out the method. It has been found that oxygen and water residues especially interfere with the surface hardening. For excluding these disruptive factors, the article is heated up to a temperature which is above the boiling temperature of water.
• a temperature comprised between 110° C. and 140° C. is preferred and 120° C. is most preferred.
• the oxygen containing atmosphere in the oven will be replaced by a first gas mixture. Therefore, the oven advantageously comprises gas inlets and gas outlets.
• the oxygen displacement will be advantageously accelerated and a possible hazard potential resulting from the contact of the standard atmosphere containing oxygen with the first gas mixture will be reduced.
• Known chemically unreactive gases such as in particular nitrogen or argon will be preferably used as inert gas.
• Non-rusting stainless steels inter alia comprise chrome as an alloying element. Due to the contact with atmospheric oxygen, a passivating and corrosion resistant chromium (III) oxide layer is formed on the material surface.
• the first gas mixture therefore has reducing characteristics, in order to avoid a further oxidation of the chrome. Furthermore, this gas mixture already initiates the depassivation of the surface.
• the first gas mixture is at least composed of a hydrogen containing gas and a nitrogen containing gas and especially preferred are H 2 and N 2 . It has been found that this gas mixture, in particular in connection with the mild temperature of the first process step, has an especially mild and advantageous effect on the chromium oxide layer without having a detrimental effect on the morphology of the surface of the filigree articles.
• the oxygen concentration will be measured continuously or at intervals by means of a sensor.
• a control unit connected to the sensor compares the actual value continuously or at intervals to a freely selectable set point and in case of an identity between the actual value and the set point, the control unit enables the oven to carry out the second process step.
• the method according to the invention is advantageously highly simplified hereby and minimizes possible sources of error for the user in this manner.
• a second process step in which the article is heated up to the target temperature, the second temperature, for the carburization.
• the second temperature is preferably selected such that this one is clearly beneath the recrystallization temperature of highly cold formed iron alloys (680° C.).
• the second temperature is preferably comprised between 450° C. and 550° C. and is most preferably 500° C.
• the heating up phase especially serves to the gentle and complete depassivation of the chromium oxide layer.
• the heat-up rate is preferably comprised between 0.5 and 1° C./min, more preferably between 0.5 and 0.7° C./min and most preferably 0.5° C./min.
• the temperature range, in which this low heat-up rate is selected is preferably comprised between 420° C. and 550° C., more preferably between 450° C. and 500° C. and most preferably between 480° C. and 500° C.
• the first gas mixture will be replaced by a second gas mixture in the second process step.
• a mild depassivation of the thin-walled deep-drawn parts during the heat-up phase to the second temperature will be preferably realized by a gas mixture that is at least composed of a hydrogen containing gas, a nitrogen containing gas as well as a carbon containing gas.
• a gas mixture that is at least composed of a hydrogen containing gas, a nitrogen containing gas as well as a carbon containing gas.
• an especially slow and thus mild and well controllable depassivation of the chromium oxide layer can be preferably achieved.
• the article will be treated with additives which selectively or entirely dissolve the passive layer.
• additives especially refer to salt compounds and/or organic substances and acidifiers which are applied to the good or in the oven in solid or liquid form.
• the application takes preferably place before the article is inserted into the oven or during the second process step.
• solids and/or liquids are used which form acid reaction products in connection with the reaction gases, which reaction products would result in a pH value of ⁇ 7 if they were introduced into water.
• the application of the substances directly onto or into the article surface has proved to be especially advantageous.
• local depassivation processes which early initiate and promote a uniform depassivation will be initiated already at low temperatures.
• carbon containing component preferably carbon oxides, saturated, unsaturated, aliphatic, cyclic, heterocyclic and/or aromatic hydrocarbons can be added to the second gas mixture.
• unsaturated hydrocarbons such as especially ethyne, is highly preferred.
• nitrogen containing component preferably elementary nitrogen, ammonia, amines, amides, imides, nitriles and/or nitrogen oxides can be added to the second gas mixture.
• the temperature will be measured continuously or at intervals by means of a sensor.
• the control unit connected to the sensor compares the actual value continuously or at intervals to a freely selectable set point for the second temperature and in case of an identity between the actual value and the set point, the control unit enables the oven to carry out the third process step.
• the method according to the invention is advantageously highly simplified hereby and minimizes possible sources of error for the user in this manner.
• a third process step is provided, in which the deep-drawn part is constantly kept on the second temperature.
• the third process step serves to the carburization of the thin-walled deep-drawn part.
• the second temperature advantageously enables a gentle formation of the surface layer to be hardened.
• the diffusion of the carbon into the surface area of the deep-drawn part takes place slowly at these temperatures, can thus be easily controlled and causes a homogenous surface layer that is rich in carbon to form.
• a too high temperature has to be avoided in any case, since due to the high diffusion speed and the high kinetic energy of the involved molecules, uneven layers and carbide particles will be formed.
• the second gas mixture will be replaced by a third gas mixture which is especially suitable for a gentle carburization under mild conditions.
• a gas mixture which is at least composed of a hydrogen containing gas, a nitrogen containing gas as well as a carbon containing gas has proved to be advantageous. It can be preferably provided to add another carbon containing component to this gas mixture, whereby the formation of a homogenous surface layer which is rich in carbon will be promoted in a synergetic manner by the two different carbon components.
• first carbon containing component preferably carbon oxides, saturated, unsaturated, aliphatic, cyclic, heterocyclic and/or aromatic hydrocarbons can be added to the third gas mixture.
• unsaturated hydrocarbons such as especially ethyne, is highly preferred.
• a second carbon containing component preferably carbon oxides, saturated, unsaturated, aliphatic, cyclic, heterocyclic and/or aromatic hydrocarbons can be added to the third gas mixture.
• carbon oxides such as especially carbon monoxide, is most preferred.
• nitrogen containing component preferably elementary nitrogen, ammonia, amines, amides, imides, nitriles and/or nitrogen oxides can be added to the third gas mixture.
• the individual concentrations of the gas components will be measured continuously or at intervals by means of respective sensors.
• the control unit connected to the sensors compares the respective actual values continuously or at intervals to freely selectable set points for the respective concentration of the gas component and compensates deviations within a fault tolerance continuously or at intervals.
• the process control is advantageously simplified hereby and allows providing constant process conditions, which is of decisive importance for the formation of a homogenous surface layer rich in carbon.
• the layer thickness of the surface layer rich in carbon can be set by means of the duration of gassing.
• a period of time comprised between 2 and 10 hours is required for generating a surface layer having a thickness of 10-40 ⁇ m.
• control unit which comprises a corresponding device for measuring the time, will enable the oven to carry out the fourth process step after a freely selectable carburization time has elapsed.
• the method according to the invention is advantageously highly simplified hereby and minimizes possible sources of error for the user in this manner.
• a fourth process step in which the deep-drawn part is cooled down to a third temperature.
• it is preferably provided to cool down the deep-drawn part to a temperature comprised between 50° C. and 80° C. and most preferably to 60° C.
• the selection of the atmosphere in which the cooling down process takes place is of decisive importance for the formation of a homogenous surface layer. It is therefore provided according to the invention to replace the third gas mixture by a fourth gas mixture.
• the selection of a slightly reducing gas mixture is especially considered to be advantageous.
• the fourth gas mixture is composed of at least a hydrogen containing gas and a nitrogen containing gas.
• the fourth gas mixture is composed of H 2 and N 2 .
• the composition of the fourth gas mixture advantageously contains 5% to 25% H 2 and 75% to 95% N 2 , more preferred 5% to 10% H 2 and 90% to 95% N 2 and especially preferred 5% H 2 and 95% N 2 . It has been shown that the cooling down according to the invention of the deep drawn part effectively prevents an escape of the carbon from the hardened surface layer.
• the invention furthermore relates to a surface hardened deep-drawn article having very small wall thicknesses.
• the deep-drawn article according to the invention comprises a soft elastic core having a hardness comprised between 350 and 400 HV1 and a hard surface layer rich in carbon.
• the surface layer is free of defect sites and/or particles, completely closed over the circumference and comprises an essentially flat surface.
• the thin-walled deep-drawn article according to the invention comprises mechanical properties of a hitherto unattained quality.
• the deep-drawn article according to the invention comprises a surface area comprising a layer rich in carbon and having a hardness of 700 to 1000 HV0.01 and a layer thickness comprised between 10 and 40 ⁇ m.
• the corrosion and the abrasion resistance of the deep-drawn article are better than the ones of the starting product.
• the first aspect is surprising in so far as a carburization usually deteriorates the corrosion properties of a steel product.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
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• Organic Chemistry (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)
• Heat Treatment Of Articles (AREA)

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Citations (13)

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• 2013
  • 2013-12-06 PL PL13196076T patent/PL2881492T3/pl unknown
  • 2013-12-06 EP EP13196076.7A patent/EP2881492B1/de active Active
• 2014
  • 2014-12-02 US US14/557,574 patent/US9738962B2/en active Active
  • 2014-12-04 CN CN201410737053.1A patent/CN104451534B/zh active Active

Patent Citations (13)

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